Project Summary Estrogen receptor (ER) positive luminal breast cancers account for nearly 75% of cases and frequently contain a wide range of progesterone receptor (PR) positive cells. While endocrine therapies directed at blocking ER action are highly effective, up to 1/3 of patients eventually progress to metastatic disease. ER and PR are key mediators of paracrine (i.e. between cells) signaling events. We predict paracrine signaling in response to aberrant steroid hormone receptor (SR) action profoundly impacts breast tumor progression, in part by modulation of the local environment surrounding a developing tumor. An emerging biomarker of increased breast cancer risk and poor prognosis in patients with invasive ER+ breast cancer is cytoplasmic (cyto-) PELP1, a normally nuclear SR co-activator. Cyto-PELP1 amplifies proliferative signaling pathways by unknown mechanisms. Our group discovered that ER and PR-B collaborate with PELP1 in novel signaling and transcriptional complexes to regulate global changes in estrogen-induced ?PELP/ER/PR? target gene expression associated with advanced tumor behaviors and endocrine resistance. Herein, we hypothesize shuttling/mislocalization of PELP1 to the cytoplasm acts as an early oncogenic event that activates direct signaling pathway inputs to nuclear ER and/or PR action, leading to altered transcription programs that favor hormone-induced tumor development and rapid progression. To address this research question, our Aims will determine: 1) the impact of cyto-PELP1/ER/PR complexes as mediators of altered hormone-driven SR target gene expression, particularly of hormone-induced paracrine pathways modulating the microenvironment, 2) the requirement for select cyto-PELP1 binding partners in altered SR actions stemming from PELP1 dynamic shuttling and mislocalization to the cytoplasm, and 3) how cyto-PELP1 contributes to hormone-induced tumor formation and progression in vivo. This research plan will span modern molecular, signaling, epigenetic, and genetic techniques, and employ complementary in vitro and in vivo models. The proposed training plan will extend the applicant's technical and theoretical breadth and depth, and is balanced with appropriate expertise, collaborators, and mentoring that includes extensive career development. Overall, this proposal will provide the applicant with a solid foundation to transition into future independent work focused in cancer research. In the short-term, this project will yield increased clarity and insight regarding the context-dependent actions of cyto- PELP1/ER/PR-containing complexes in modulating ER+ luminal breast cancer development. For example, the use of novel gene signatures (developed herein) associated with cyto-PELP1 may be a useful tool to identify patients at high risk for recurrence while on long-term endocrine therapy, and reveal novel strategies aimed at preventing breast cancer development.